A hybridized orbital of a semiconductor material has a conductive band, and its electrons of the valence band will be promoted across the band gap to the conductive band by absorbing an appropriate energy.
Photocatalysis reaction is performed to create electron-hole pairs by radiating a semiconductor material, and the electron-hole pairs will generate radicals (e.g. hydroxyl radicals: .OH) able to undergo secondary reactions, such as pollutants removal, deodorization and disinfection. However, the band gap of the semiconductor materials, which can be used as an environmental photocatalyst, must fit in with the energy range of the radiation source so that the electrons can be promoted across the band gap to the conductive band via the abovementioned radiation and further generate redox ability.
Titanium dioxide occurs in nature as well-known minerals rutile, anatase and brookite and is extensively applied in photocatalysis reaction due to its excellent structure and properties. The brookite-type phase is unusable due to its instability. Furthermore, although the recombination rate of e−/h+ pairs of the rutile-type phase is faster, but the present method for fabricating the rutile-type phase is always performed by a calcination process. Therefore, the rutile-type phase has poor photocatalysis capability, or has no photocatalysis capability.